Extraterrestrial life in the Solar System

Exobiology is the search for and study of life on celestial bodies other than Earth. Within the solar system, Mars, Jupiter’s satellite Europa, and Saturn’s satellites Titan and Enceladus are considered among the most possible sites where life, or the precursor chemistry needed for the rise of primitive living organisms, might have developed.

Overview

Understanding where life might have developed in the solar system requires comprehension of how life arose on Earth. The earliest evidence of life on Earth is the presence of organic matter derived from biological processes recorded in rocks that are about 3.2 billion years old. Life may have developed very early in Earth’s history. However, much of the fossil record of early life on Earth has been erased by subsequent geophysical activity. Biologists have pieced together some of that early history by examining the remaining fossil record and by performing a series of laboratory experiments.

Life on Earth is based on complex organic molecules consisting of chains of carbon, hydrogen, nitrogen, and oxygen. However, organic molecules can be produced by simple chemical reactions and by biological activity. Thus, to determine if a process is truly biological rather than simply a chemical reaction, it is necessary to define the criteria for life. The ability of an organism to reproduce itself is considered an essential feature of life. Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are the organic molecules that control heredity in terrestrial life forms. Thus, DNA and RNA are considered essential for the reproduction of life on Earth. These two nucleic acids are produced only with the help of certain proteins. A major focus of exobiology is to understand how DNA, RNA, and the proteins essential in their production originated.

A major breakthrough occurred in 1953, when Stanley Miller, a graduate student at the University of Chicago, and his research supervisor, Professor Harold Urey, produced amino acids, the basic building blocks of proteins, in a sealed environment simulating conditions believed to be present on the early earth. Miller and Urey continuously passed electrical sparks through a chamber filled with a gaseous mixture of methane, ammonia, and hydrogen (a composition believed to be similar to that of the early atmosphere of Earth) and water vapor (representing the water contributed by Earth’s oceans). After several days, they extracted a mixture of organic molecules, including amino acids, from the bottom of the chamber. The Miller-Urey experiment suggested lightning discharges throughout Earth’s early atmosphere could have deposited amino acids onto the planet’s surface.

Other experiments demonstrated that bombardment of the gas mixture by high-energy particles, simulating cosmic rays, produced similar results. Though controversial (for example, later studies have suggested that Earth's early environment was markedly different from the experimental conditions), these experiments suggest that three sufficient conditions must be met to produce amino acids: a supply of carbon-rich material must be present, liquid water must be available, and some energy source (electrical discharge, high-energy particles, or possibly heat and sunlight) is required. Although sufficient, it remains to be proven that these three conditions are also necessary. Some scientists, however, doubt that the formation of organic molecules can necessarily lead to the development of life.

Scientists have examined the planets and satellites of the solar system, searching for locations where all three conditions are met. Water may be the most critical restriction since it remains a liquid only over a very narrow range of temperatures. The surfaces of Venus and Mercury are too hot for liquid water to be present. Jupiter, Saturn, Uranus, Neptune, and Pluto are too cold to support liquid water. Thus, of the planets, only Earth and Mars seem to be suitable candidates for life because they are in the range of distances from the Sun such that they could support liquid water. Venus is within the Sun’s habitable zone as well, but it was believed that a runaway greenhouse effect of unknown origin has left it largely inhospitable to life as people understand and recognize it. Therefore, much exobiological research and speculation has instead focused on various moons rather than the planets themselves.

Scientists have gone back and forth over whether Mars may support life or did so in the past, with consensus shifting as new evidence is made available. One method of study involves looking for secondary evidence of life, a technique used to observe distant exoplanets as well as Mars. Where life is abundant, it can produce changes in the atmosphere of a planet, allowing astronomers to search for unusual signatures of biological activity. The present composition of Earth’s atmosphere, dominated by nitrogen and oxygen, is regulated by the life cycle processes of respiration and photosynthesis of Earthly organisms. The atmosphere of Mars, on the other hand, is dominated by carbon dioxide, and it contains only a trace amount of oxygen. Thus, by the 1960s, astronomers had observed that, at least in the present era, living organisms were not present in sufficient abundance to perturb the atmospheric chemistry of Mars.

The beginning of the space age made it possible to employ robotic spacecraft to perform direct measurements on the surface of some planets in the expanding search for evidence of life. The first search was performed on Mars by two Viking spacecraft, developed by the National Aeronautics and Space Administration (NASA), which landed safely in 1976. Each Viking spacecraft was equipped with instruments designed to examine the soils of Mars for evidence of Earth-like life. Though no direct evidence was found, subsequent missions continued to boost exobiologists' hope that at least the conditions for life once existed on Mars. By 2015, scientists had confirmed the presence of liquid water on the planet, further suggesting that some kind of microscopic life could exist there. In 2020, scientists published a paper claiming that, following the 2018 report of the detection of a possible lake beneath the planet's surface, they had analyzed a large amount of data received from the Mars Express orbiter that indicated the presence of a system of lakes under the surface made up of that lake as well as at least three additional lakes. However, more studies must be conducted to determine the salinity levels of the suspected bodies of water. The Mars Express continued to collect data in the early 2020s as it entered its third decade of operation.

The search for extraterrestrial water remained key to the search for extraterrestrial life as known on Earth, and by the 2010s, scientists had confirmed or hypothesized the presence of liquid or frozen water on several moons, including Jupiter’s Europa and Saturn’s Titan and Enceladus. These satellites, thus, became the chief subjects of exobiological research, with evidence suggesting the presence of large, relatively warm oceans beneath shells of ice—considered prime locations for potential life. In the late 1990s and throughout the first decades of the twenty-first century, extrasolar planets were also increasingly detected. Although most of the first hundred or so worlds were hot Jupiter-like giants or at least bizarre large planets in systems not conducive to life as people understand it, in time, technology became capable of picking up Earth-sized planets. A space-based observatory named Kepler was readied for launch in late 2008. Data from Kepler greatly expanded the list of extrasolar planets, including detecting the first Earth-like planets. It also found that many of the larger gas planets had rocky or icy moons similar to Jupiter's that could sustain life.

While scientists improved their chances of detecting Earth-like, carbon-based life forms reliant on water, they also broadened their understanding of how life could be radically different. During the 1980s and 1990s, developments in terrestrial biology changed how exobiologists looked at the essential conditions to develop life. Single-celled organisms called archaebacteria, which may have developed very early in Earth’s history, were discovered. These organisms live in oxygen-deprived places, such as the hot springs or even tar pits. Archaebacteria take in carbon dioxide and give off methane, and they actually cannot thrive in the presence of oxygen. They have genetic material different from that of other terrestrial life forms, suggesting that they possibly evolved independently from the more common life forms very early in Earth’s history at a time before the oxygen-rich atmosphere arose. Other terrestrial microorganisms were discovered that live on sulfur from geothermal sources rather than relying on the Sun to supply energy. The discovery of these unusual terrestrial life forms suggests that conditions required for the development of the common life forms on Earth may not be required for the development of all life. Thus, some planets and/or their satellites previously believed to be unsuitable for the development of life may be habitable by organisms rather different from the common life forms on Earth.

Such discoveries complicate the search for extraterrestrial life because many experiments, such as those conducted by the Viking spacecraft on Mars in 1976, look only for signatures specific to common terrestrial life. While exobiologists can broaden their search and pay attention to environments, such as thermal vents, the possibility has been raised that life encountered on another planet or moon might be so foreign to known science that it may not be recognized as life. For example, some researchers have proposed that there is no reason life would have to follow the same conditions as on Earth if it developed under drastically different conditions. A non-carbon-based life form or one that evolved in the absence of water, for example, in the liquid methane seas theorized to exist on Titan, might be so alien as to be unimaginable. Such discussions around the complexity of the concept of and conditions for life forms were heightened in 2020 when a team of astronomers and scientists hypothesized about the potential for a life form to exist within Venus's atmosphere. Based on data they accumulated through an observatory and a high-powered telescope on Earth, they claimed to have detected a phosphine signature in the planet's clouds. As it was believed that the presence of the gas on or around such a rocky planet was an indicator of life as it is typically produced by living organisms, and they reported having modeled other possibilities, they claimed that a life form was the most scientifically sound reason behind the detection of the gas. Additionally, they pointed out that the position within the clouds at which the detection was made had been theorized to be at lower temperatures, more sustainable for life. While the implications of this hypothesis were recognized and interest in the idea of Venusian life was revived, many researchers remained incredulous as more needed to be understood about the planet's makeup and processes.

Methods of Study

One focus of the search for life is to identify the carbon-rich compounds available for life’s development. Impacts of meteorites, asteroids, and comets are believed to have contributed a carbon-rich layer to the Earth’s early surface and other planets and their satellites. One particularly carbon-rich meteorite, called Murchison, fell in Australia in 1969. Detailed studies of Murchison established that it contains numerous organic compounds, including amino acids.

In 1986, five spacecraft, two launched by the Soviet Union, two by Japan, and one by the European Space Agency, flew past Halley’s comet. Dust analyzers on some of these spacecraft determined the chemical composition of individual dust particles emitted by the comet. These instruments detected a large number of carbon-rich particles, many of which also contained hydrogen, suggesting the presence of organic molecules in the dust. However, detailed analysis of organic molecules requires sophisticated scientific instruments too large and complicated to be flown on those spacecraft. NASA launched a spacecraft called Stardust to fly to Comet Wild 2 to collect dust emitted by that comet. It successfully returned samples to Earth in 2006. Laboratory study of the dust established the abundances and types of organic compounds present in Wild 2.

The second focus of the search for life is to perform direct tests for the presence of biological activity on other planets or satellites. Apollo astronauts collected the first samples from the Moon in 1969. When they returned to Earth, the astronauts, their spacecraft, and their prized lunar rocks were subjected to a twenty-one-day quarantine during which scientists searched for living microorganisms that might be hazardous to life on Earth. Fragments from lunar rocks were crushed and placed in a standard culture medium, a nutrient-rich soup that promotes the growth of microorganisms. Microscopic examination of these samples showed no evidence of living microorganisms. More detailed studies of the lunar rocks have shown no fossil evidence of life forms that might once have developed on the Moon but are now extinct. Examination of lunar samples revealed them to be exceptionally dry, with none showing any evidence of liquid water. The absence of liquid water was taken to indicate that the Moon was always a lifeless body.

Initial experiments in the search for life on another planet were conducted in 1976 by the two Viking spacecraft that landed on Mars. Each Viking carried four instruments to examine the soil samples for evidence of such basic life-cycle processes as respiration or photosynthesis. The Gas Exchange Experiment deposited samples of Martian soil in a chamber containing a culture medium. This apparatus monitored the composition of gas within its chamber, looking for changes in the abundance of carbon dioxide, oxygen, or hydrogen that would signal metabolic activity by microorganisms in the soil.

In a second experiment, the Labeled Release Experiment, radioactive carbon atoms were incorporated into the culture medium. A detector looked for the appearance of radioactive carbon in released gas, signaling that the addition of Martian soil to the nutrient had resulted in a reaction of biological origin. Both experiments produced positive results, but the effects were much more dramatic than the scientists had expected. These positive results were eventually explained as chemical reactions initiated because of the highly reactive nature of the surface materials on Mars resulting from their exposure to ultraviolet light from the Sun, a superoxide chemical reaction.

The Pyrolytic Release Experiment provided an opportunity to test that explanation. It was also a labeled release experiment, but this apparatus had the additional capability of heating soil samples between experiments. Scientists heated soil to 548 kelvins, well above the temperature expected to kill any microorganisms present in the soil. Even then, the Pyrolytic Release Experiment yielded positive results, suggesting that the release was produced by a chemical reaction involving superoxides rather than a biological process.

A fourth experiment, the Gas Chromatograph Mass Spectrometry Experiment, produced the most convincing evidence that the soils at the Viking landing sites contained no microorganisms. This instrument found no organic molecules within the soil down to a limit of a few parts per million. Even the organic molecules that would be expected in the soils from the accumulation of meteorites like Murchison were absent. Subsequent studies indicated that the high chemical reactivity of the soils and intense ultraviolet radiation striking the surface would rapidly destroy most organic molecules. Thus, if there is life on Mars, the two Viking spacecraft, which could only sample the near-surface soils, were probably looking in the wrong places.

Although instruments on both Viking landers found no evidence of biological activity in their soil samples, the two Viking orbiters obtained high-resolution photographs of Mars’s surface, producing results that excited exobiologists. Several regions on Mars revealed features similar to extensive water flow channels on Earth, leading many geologists to conclude that water had flowed freely on the surface of Mars at some earlier period in its history. Later missions found ice deposits and eventually even liquid water on Mars. Because of the assumed importance of liquid water in the development of life, some exobiologists suggested that life might have developed on Mars in that earlier era and that life might exist in subsurface layers protected from ultraviolet radiation; this theory gained additional support after the 2020 report regarding the detection of what seemed to be several subsurface lakes. Or perhaps such life had gone extinct, leaving only fossil evidence behind.

In 1996, scientists from NASA’s Johnson Space Center reported that a meteorite called ALH 84001, one ejected from the surface of Mars and deposited in the Antarctic about thirteen thousand years ago, contained microscopic features that might indicate ancient Martian biological activity. This resulted in renewed interest in the search for life on Mars. These suspected fossils resembled wormlike creatures, but their size was extraordinarily small. Many scientists pointed out, however, that similar nanometer-sized structures could be produced geochemically and had nothing to do with life.

After the 1997 Mars Pathfinder exploration of the Red Planet returned amazing images of rocks and terrain, NASA planned a series of robotic spacecraft to continue the exploration of Mars. Two of those spacecraft, the Mars Exploration rovers named Spirit and Opportunity, launched in June and July 2003, respectively. They successfully landed on Mars in early 2004 and spent at least the next four years moving about their landing sites, searching for evidence of water. The Curiosity rover, which launched in 2011, contributed findings over time that provided scientists with further insight into the potential history of water on the planet.

The same techniques used to search for existing or fossil life on Mars can be applied to other planets or satellites that are identified as suitable candidates for the development of life. The Galileo spacecraft, placed in orbit around Jupiter in late 1995, obtained close-up photographs of Jupiter’s four largest satellites. One of these, Europa, emerged as another potential site for the development of life. One of Galileo’s orbits around Jupiter took it within 363 miles of Europa’s surface, allowing its cameras to photograph objects as small as seventy-five feet across. These images showed evidence of ice flows that had broken from a solid sheet and been displaced, suggesting that they had floated or slipped across a liquid ocean or on a layer of slush below. Calculations indicated that Jupiter’s extreme gravitational pull could introduce tidal distortions that produce sufficient heat to allow liquid water to exist beneath Europa’s icy surface. Other photographs showed dark deposits, possibly carbon-rich material contributed by meteorites.

An observation by the Hubble Space Telescope in 2013 identified a spout of water vapor from the surface of Europa, generating further interest in the possibility of a liquid ocean beneath the satellite's ice shell. Exobiologists were excited to see the possible existence of the three conditions believed necessary for the development of life: carbon-rich material, water, and energy from the Jovian tides. These findings led NASA to plan a mission to further study Europa as a prime candidate for extraterrestrial life. A spacecraft placed into orbit around Europa could use radar to see through several miles of ice, detecting any water below and providing a clear test of the ocean model. More ambitious proposals have included a spacecraft that would fling a nine-kilogram projectile into the surface of Europa, catch some of the debris lofted by the collision, and return it to terrestrial laboratories for examination. Another common proposal would see a submersible vehicle melt its way through Europa’s icy crust to reach a potential subsurface layer of liquid water and image the local environment directly.

Titan, the largest satellite of Saturn, has a methane-rich atmosphere believed to be similar in composition to that of the early Earth. High-energy electrons and protons, trapped in the magnetic field of Saturn, continually bombard the upper region of Titan’s atmosphere. This bombardment is believed to produce complex organic molecules that rain down onto Titan’s surface. Titan is too cold to have liquid water. Titan remained the primary target of study for the Cassini spacecraft, which was launched in October 1997 and arrived in the Saturn system in early July 2004. Cassini dropped its Huygens probe, loaded with instruments to measure the types and abundances of the organic molecules, into Titan’s atmosphere. The Huygens probe showed its surface may be covered with lakes of methane or ethane, which some scientists speculate might be sufficient to allow primitive life to develop. Also, Titan’s crust appears to move significantly as if floating on a subsurface ocean, adding another intriguing aspect to the possibility of organic chemistry and/or primitive life on Titan.

Even Saturn's smaller satellite, Enceladus, displays unexpected geyser activity at its south polar regions. This suggested the possibility of liquid water underneath the surface and, therefore, the potential for primitive life. The presence of an ocean of salt water beneath the Moon's ice crust was confirmed in 2014, and data about the Moon captured by Cassini continued to be studied even after the mission ended in 2017, with a team of researchers arguing in 2018 that such data included indications of the existence of bigger, more complex organic molecules. Neptune’s Triton also exhibits cryovolcanism at an even lower temperature. More research is needed to determine the nature of this mechanism, and that investigation would likely have to await a Neptune orbiter.

Context

The possibility that life might have developed elsewhere in the solar system has been the subject of speculation for hundreds of years. In 1820, Carl Gauss, a German mathematician, suggested cutting geometrical patterns into the Siberian forest large enough to be seen by an observer using a telescope from the Moon or Mars. The idea was to motivate any inhabitants of the Moon or Mars to engineer similar geometrical patterns, initiating crude communication with Earth. Other suggestions for communication with extraterrestrial intelligent life included setting huge fires in the Sahara desert and constructing large mirrors to reflect sunlight into space. These early ideas of communicating with intelligent life elsewhere in the solar system did not focus on particular sites where the conditions were expected to be appropriate for the development of life.

Although its origins go back as far as 1929, radio astronomy only gained respect within the astronomical community in the early 1950s in the aftermath of World War II, when radio equipment necessary to "listen" to the heavens became available as war surplus. Radio astronomers soon discovered that the natural universe was far from radio-quiet. Some scientists, beginning with astronomer Frank Drake, wondered about and then tested the idea that intelligence beyond Earth might transmit recognizable radio signals. In due time, a coordinated Search for Extraterrestrial Intelligence (SETI) program was developed. No verifiable signals of intelligence have yet been received from deep space.

Only in the second half of the twentieth century did biologists begin to develop an understanding of how life originated on Earth. This knowledge provided clues as to the conditions needed for similar forms of life to develop elsewhere in the solar system. The study of terrestrial life indicates that it originated as simple, single-celled microorganisms and that these simple microorganisms might develop quickly and easily on other planets and/or their satellites as well. Thus, the focus of solar system exobiology shifted from the search for intelligent life, which has not been seen on any planet other than Earth, to the search for simple microorganisms. However, SETI continued, although, for a time, Congress removed any support for the project through NASA’s federal allocations. In time, commercial funds supplemented federal funding for SETI projects. For a time following the release of the movie Contact (1997), based on a book by astronomer and author Carl Sagan, public interest in SETI increased dramatically.

The dawn of the space age inaugurated an era when spacecraft could be used to search for environments favorable to the development of life, perform experiments designed to detect living organisms on the surface of other planets and/or their satellites, and ultimately return samples to Earth so that scientists could examine them for evidence of biological activity or fossil evidence of past life. Although scientific interest in life elsewhere in the solar system reached a low point after the negative results of the Viking landers in 1976, there was a resurgence of interest by the end of the twentieth century. Discovery of river channels on Mars, possible fossil evidence for ancient microorganisms in a meteorite from Mars, hints of water ice on the Moon and Mercury, oceans on Europa and Enceladus, organic materials and an atmosphere on Titan, and cryovolcanism seen on Enceladus and Triton suggest that the solar system might not be as inhospitable to the development of life.

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